1. Field of the Invention
The present invention relates to a rotation detection apparatus including a magnetic-electric conversion element to detect a rotation state of a gear.
2. Description of Related Art
Japanese Patent No. 3988316 discloses a magnetic sensor that includes a bias magnet and a sensor chip having a magnetoresistive (MR) element. The bias magnet applies a bias magnetic field to the MR element. The magnetic sensor further includes an electric current conduction part disposed on a substrate on which the MR element is arranged. According to the above magnetic sensor, the electric current is applied to the electric current conduction part to adjust a direction of the bias magnetic field acting on the MR element. It is thereby possible to adjust a sensor output offset, which can result from a bias magnet magnetization variation and a relative positioning error between the MR element and the bias magnet etc.
JP-A-2003-254780 discloses a rotation detection apparatus that has multiple magnetic sensors. The multiple magnetic sensors perform mutual monitoring and self-diagnosis thereof to determine whether the magnetic sensor has a malfunction that fixes an output voltage to a power supply voltage, a ground voltage, a middle voltage or the like.
According to the magnetic sensor disclosed in Japanese Patent No. 3988316, a sensor output offset of the magnetic sensor can be adjusted in accordance with the above-described manner. However, when a detection target object is in a stationary state, the magnetic sensor cannot perform self-diagnosis thereof to determine whether there exist an abnormality, e.g., the decrease in sensitivity or the presence of the offset due to a failure or aging of the MR element and the bias magnet.
According to the rotation detection apparatus disclosed in JP-A-2003-254780, it is possible to perform self-diagnosis to determine whether there exists a malfunction that fixes a sensor output signal to a certain voltage. However, the rotation detection apparatus cannot perform self-diagnosis to determine whether there exists a failure, e.g., the decrease in sensitivity or the presence of the offset caused by failure or aging of the MR element and the bias magnet. Further, since the rotation detection apparatus includes multiple magnetic sensors, manufacturing cost is high.